Cadmium-Induced Responses and Tolerance Mechanisms of Aerobic Methanotrophs in Rice Paddy Soils

Paddy fields are major sources of atmospheric methane and are at risk of cadmium (Cd) contamination. Aerobic methanotrophs, which serve as biological methane sinks, play a key role in methane cycling, but their responses to Cd stress remain poorly understood. Here, we examined the relationship between Cd pollution levels and aerobic methane oxidation potential in paddy soils. We evaluated methanotrophic enrichments under Cd exposure, applied metagenomic sequencing to identify functional microbes, and investigated Cd tolerance mechanisms in pure culture. Aerobic methane oxidation rates were positively correlated with Cd levels in paddy soils from South China, with Methylocystis and Methylomonas emerging as dominant genera possessing diverse Cd tolerance genes. Notably, interspecific differences in Cd tolerance were observed among methanotrophic strains. The faster-growing Methylomonas sp., endowed with more robust antioxidant defenses and extracellular polymeric substances synthesis genes, exhibited Cd resistance through markedly enhanced loosely bound extracellular polymeric substances production, in contrast to the Cd-sensitive Methylobacter sp. Gene knockout experiments confirmed the essential roles of glutathione synthase, glutathione peroxidase, and exosortase in exopolysaccharide extrusion for Cd detoxification. These findings advance our understanding of the methane cycle in Cd-contaminated rice paddies and suggest potential strategies to mitigate methane emissions while addressing Cd detoxification.